The present invention relates to the field of fabricating ohmic contacts on a semiconductor, and more particularly to fabricating ohmic contacts on semiconducting diamond.
The possibility of growing epitaxial layers of semiconducting diamond has generated a resurgence of interest in developing applications of this unique material for microelectronic devices. That uniqueness is, perhaps, best exemplified by the fact that diamond possesses high elastic modulus, high thermal conductivity, low dielectric constant, optical transparency over a wide spectral region, and a superior resistance to chemical attack and to thermal or radiation damage. Further, based on theoretical predictions, exceptionally high saturated carrier velocities for both electrons and holes over a wide range of applied voltages may be achievable in diamond devices. These exceptional properties suggest the potential of diamond as a material useful in severe environments where present state-of-the-art semiconductor devices are not practical or effective, as for example, high temperature (&gt;400.degree. C.) environments, chemically corrosive environments, outer space, and environments subject to radiation exposure, as are found in nuclear reactors.
Semiconductor devices require ohmic contacts to provide reliable operation and controlled outputs. Therefore, a process that could produce ohmic contacts on semiconducting diamond and be implemented on a mass production basis would enable semiconducting devices to be employed in many new applications.
Contacts have been formed to both natural and synthetic diamond for a variety of measurements of the physical properties of these materials, as for example, photoconductivity and electrical transport. Further, the utilization of diamond for nuclear radiation detectors, light sensitive switches, and high temperature thermistor elements has required the fabrication of contacts onto diamond. Three methods have been used to fabricate such contacts, but each has performance and other limitations which makes them unsuitable for use in a mass production environment or for utilization at elevated temperatures.
The first method requires a roughened diamond surface. When metal is placed in contact with a mechanically damaged area of diamond such as a crack, a corner, or a region that is deliberately roughened, acceptable electrical contact may be attained. Electrical transport and infrared photoconductivity measurements were made successfully using colloidal graphite or silver paint contacts on roughened diamond surfaces, or point contacts on the edge of a polished diamond block. However, silver paint and colloidal graphite contacts are mechanically fragile and electrically noisy. The rough surface features and the large concentrations of surface defects that were present on the early synthetic semiconducting diamonds allowed electrical transport measurements to be made using point contacts. These contacts were reported to be non-ohmic and partially rectifying, but far superior to similar contacts formed on natural diamond. However, contacts produced by this method have poor reliability and lack mechanical integrity, making them unsuitable for use in adverse environments.
The second method relies on the reaction of the diamond with a molten alloy. It has been suggested that transition metal/corrosion resistant metal alloys, in the molten state, will etch the diamond surface, form a low resistivity transition metal carbide and diffuse the corrosion resistant metal into the diamond. This technique was used to form thermistor elements from natural semiconducting diamond with a copper-silver-titanium alloy. Gold-tantalum and gold-tantalum-aluminum alloys have also been used to form electrical contacts to natural insulating and semiconducting diamond surfaces by electron beam welding and by direct joule heating. This process provides very poor control of the contact dimensions, which necessarily must be precise and repeatable in a mass production environment.
The third method involves graphitization of the diamond surface which provides ohmic response. Various means have been proposed for the graphitization of the diamond surface and the means for making contact to the graphite layer. However, each graphite layer has different thermal and chemical properties than the underlying diamond. Therefore, this process does not produce contacts suitable for high temperature applications.
All of these processes produce contacts having resistance values which are difficult to predict, and none are compatible with standard photolithographic processes. Contacts formed by any of these methods are not able to withstand the operating conditions for which diamond devices would be potentially suitable: elevated temperatures, high current densities, high frequencies, and exposure to intense radiation. Contacts formed by these methods have high contact resistances and are noisy.
Thus, a need exists for a method capable of being implemented on a mass production scale that produces ohmic contacts on semiconducting diamond. Therefore, an object of the present invention to provide a method for manufacturing relatively low resistance ohmic contacts on semiconducting diamond. Another object of the present invention is to provide a method for producing low resistance ohmic contacts on semiconducting diamond that is compatible with conventional processing techniques. A further object of the present invention is to provide a method for producing low resistance ohmic contacts having resistances within a predictable range of values on semiconducting diamond. Still yet another object of the present invention is to provide a method for manufacturing relatively low resistance ohmic contacts on semiconducting diamond with precise and repeatable dimensional control of the contact dimensions.